This project is intended to identify mechanisms by which the nicotinic acetylcholine receptor comes to be localized to specific regions of the postsynaptic membrane, and by wich this localization is maintained. The dynamic phase of receptor localization will be studied by studied by following movements of new receptor into and within motor endplates of mammalian or chick skeletal muscle in which the receptor already present has been blocked quasi-irreversibly. For study of the static phase, the assumption is made that receptor localizations are maintained by interaction of receptor molecules among themselves and/or with non-receptor pretein which is in or on the postsynaptic membrane. The biochemically advantageous electric tissue of electric rays, a tissue in which receptor localization occurs, will be used as the source of isolated postsynaptic membranes. We have evidence for the presence of non-receptor proteins specifically associated with receptor-containing regions of these membranes. Identification of these proteins and of their interaction, if any, with the receptor protein will be attempted by biochemical, morphological, and immunohistochemical means.